Last week, retired Lt. Gen. Michael Flynn, President Donald Trump's confidant on matters pertaining to national security from June 2015 to February 2017 and his short-lived national security adviser in the White House, pleaded guilty in federal court in Washington, D.C., to a single count of lying to the FBI. Under the terms of his plea agreement, Flynn, who had faced nearly 60 years in federal prison had he been convicted of charges related to all the matters about which there is said to be credible evidence of his guilt, will now face six months.

What could have caused Robert Mueller, the no-nonsense special counsel investigating whether any Americans aided the Russian government in its now well-known interference in the 2016 American presidential election, to have given Flynn such an extraordinary deal?

Here is the back story.

During the FBI's investigation of Russian meddling in the election, it became interested in Flynn's communications with Sergey Kislyak, a KGB colonel (the KGB is now known by its post-Soviet acronym, FSB) masquerading as the Russian ambassador to the U.S.

After Trump won the presidency, Flynn became an important member of the presidential transition team. Between the election and the inauguration, Flynn spoke on the telephone with Kislyak five times. Because Kislyak was a foreign spy, as well as an ambassador, his communications with Americans were monitored by the FBI.

When Flynn agreed to be interviewed by the FBI in his West Wing office on Jan. 24, he probably did not know what the agents were looking for. Jim Comey was still the director of the FBI. Mueller had not yet been named special counsel. The FBI investigation into Russian meddling in the just-completed presidential election was in its infancy.

Prior to the interview, the FBI obtained the transcripts of Flynn's conversations with Kislyak. The conversations themselves were not illegal. On the contrary, it is expected that an incoming presidential administration will begin to reach out to foreign leaders even before the new president is inaugurated.

When the FBI interviewed Flynn, it asked him whether he had spoken with Kislyak and, if so, whether they had discussed American sanctions imposed on Russian individuals as retaliation for Russian meddling in the election. Flynn acknowledged the conversations but denied that they had been about sanctions. The two agents interviewing him knew immediately that he was lying, because they had read the transcripts of his conversations.

Since the FBI knew the subject matter of the Flynn-Kislyak conversations, what was the purpose of the Flynn interview? And given that the conversations were lawful — as long as they occurred after Trump's victory — why would Flynn lie about them? As well, given that Flynn once ran thousands of surveillance projects against high-level foreign targets, how could he not have known that the FBI knew what he had discussed with Kislyak before its agents walked into his office?